The field of the invention is in the electronic counter-measure art and more particularly that of radar absorbing materials for passive ECM.
The purpose of jamming a radar is to create deliberate interference and to degrade the radar's usefulness as part of a weapon system. The various techniques that electronically interfer with radar performance are called electronic countermeasures (ECM). Electronic countermeasures can be divided into two classes, generally known as confusion jamming or deception jamming. Both confusion and deception countermeasures may be created with either active or passive devices. Active countermeasures are those which radiate electromagnetic energy. They include noise jammers and repeater jammers. Passive countermeasures do not radiate of their own accord and include chaff, decoys, and electromagnetic absorbing materials.
Certain materials are capable of absorbing radio waves very strongly. Waves traveling in these materials will be attenuated greatly within a short distance, of the order of mills. This absorption of electromagnetic energy effectively achieves a reduction of the radar cross section of the target. As such, the return signal to the originating radar will be greatly reduced in intensity and will substantially degrade the operating effectiveness of the radar.
Ideally, the optimum radar absorbing material would be a paint-like material effective at all polarizations over a broad range of frequencies and angles of incidence. Unfortunately, such a material does not exist. Practically, the type of absorber which would be most effective in a given situation is highly dependent upon the radar frequency, target shape and dimensions, bandwidth required, and the physical constraints such as weight, thickness, strength, environment, etc., which are placed on the absorber.
Attempts to achieve the greatest amount of absorption within such constraints has led to the use of carbonyl iron particles within a dielectric material as the most effective radar absorbing material. Typically, these iron particles are uniformly distributed throughout the material with approximately equal interparticle spacing. The objective of this technique is to fill or load the dielectric material with the maximum number of carbonyl iron particles possible while maintaining a small but required spacing between the particles. Such spacing results in a homogeneous mixture of particles within the material while providing the electrical insulation necessary to accomplish the absorption of electromagnetic waves.
One of the chief environmental constraints affecting radar absorbing material is temperature. The frictional forces that are encountered due to the speed of today's military aircraft create extremely high temperatures on the skin of the aircraft. Radar absorbing material employed on such aircraft must be engineered for such heat. For instance, the typical dielectric material of plastic that is used for low temperature applications now is replaced by a ceramic material that can better accommodate the high temperature environment. One temperature related problem has continually baffled engineers however. This is the problem of oxidation of the carbonyl iron particles within the material. The high temperatures and resultant heat causes the unprotected iron particles to oxidize very fast and renders them worthless as an absorber material. The deterioration in the radar absorbing properties of this material caused by the rapid rate of oxidation results in an increase in vulnerability of the aircraft to radar guided threats, not to mention the tremendous waste of time, energy, and money in formulating and applying the then worthless absorbing material.